1. Field of the Invention
The current invention relates to battery-powered devices, and in particular, to the operation of battery-powered devices subjected to varying temperatures.
2. Description of the Related Art
Numerous consumer electronic devices are powered by batteries. Batteries allow a consumer electronic device to be mobile, i.e., to be operational away from dedicated sources of electrical power, such as wall sockets. Various battery technologies exist. Some technologies provide for single-use batteries while other technologies provide for rechargeable use. Rechargeable batteries are commonly used in mobile computers, mobile phones, cameras, handheld computers, handheld music players, and other consumer electronic devices.
One common rechargeable battery technology is lithium-ion (Li-ion). One type of Li-ion battery is the Li-ion polymer battery, also known as Li-Poly, Li-Po, or LiPo. Li-ion batteries have several advantages over other rechargeable battery technologies, such as superior energy density by weight, and the ability to be molded into a variety of shapes, including relatively thin and/or irregular shapes. Li-ion batteries are also capable of being charged and discharged at relatively high currents. Li-ion batteries have some drawbacks, such as their susceptibility to depletion damage, overheating and combustion, and their consequent need for safety and control circuitry. Other battery types may also be provided with safety and/or control circuitry.
The safety and control circuitry is typically capable of determining the temperature of the battery. One way to determine the battery temperature is to use a thermistor sufficiently close to the battery such that the thermistor is consistently at approximately the same temperature as the battery. Alternatively, the temperature of the battery may be determined by circuitry that measures particular properties of the battery, such as the resistivity of the battery, where the battery temperature is derived based on a known relationship with the measured properties. Other ways of determining the battery temperature are also possible. Consumer batteries typically operate best at an ambient temperature of around room temperature, i.e., 20-25° C. However, the operating range of Li-ion batteries is greater. For example, in certain mobile technologies, the operating range of the Li-ion batteries is expected to be −10 to +55° C.
The temperature of a battery affects the battery's operating chemistry, and thus its rates of electric current charge and discharge. As the temperature of a typical Li-ion battery decreases, its internal resistance increases. At temperatures below freezing (i.e., below 0° C.), the discharge capacity of a typical Li-ion battery is significantly reduced compared to its capacity at room temperature. The charging of a Li-ion battery is also affected by the battery temperature.
FIG. 1 shows a simplified block diagram of mobile consumer electronic device 101. Mobile consumer electronic device 101 comprises battery 102, battery safety circuitry 103, and consumer electronic device circuitry 104. Battery 102 comprises one or more electricity-producing chemical cells, such as Li-ion cells, and is preferably rechargeable. Battery 102 may be packaged in a casing that is adapted to be removed and/or replaced by a consumer. One example of such an embodiment is the battery for a typical mobile telephone. The casing containing battery 102 typically includes at least a portion of battery safety circuitry 103. Such a battery is sometimes referred to as a “smart battery.” Battery 102 may also be packaged in a casing that is not designed for removal and/or replacement by a consumer, in which case the battery casing also typically includes at least a portion of battery safety circuitry 103. Such a battery can be considered an integrated component of consumer electronic device 101. One example of such an embodiment is a portable personal music player, such as the iPod produced by Apple Computer, Inc., of Cupertino, Calif.
Battery safety circuitry 103 is designed to prevent the depletion damage, overheating, and combustion of battery 102 by regulating the electrical current into and out of battery 102. Battery safety circuitry 103 typically includes circuitry that determines the temperature of battery 102. At least a portion of battery safety circuitry 103 is typically encased with battery 102, and would thus be removed from mobile consumer electronic device 101 if battery 102 were removed. The rest of battery safety circuitry 103 may be located with consumer electronic device circuitry 104, and thus would remain in mobile consumer electronic device 101 if battery 102 were removed.
Consumer electronic device circuitry 104 comprises the circuitry of consumer electronic device 101 that provides desired electronic functionality to mobile consumer electronic device 101, is not encased with battery 102, and is not battery safety circuitry for battery 102. Consumer electronic device circuitry 104 typically comprises a controller for mobile consumer electronic device 101, such as controller 105, and can include means for providing information to a user, i.e., a human-machine interface (HMI), such as a display screen, a speaker, or LEDs (not shown). Controller 105 interacts with battery safety circuitry 103 to get electrical power from battery 102 via battery safety circuitry 103.
Consumer electronic devices are not typically used at temperatures below freezing because consumers generally prefer to avoid exposing themselves, and thus the personal electronic devices they have with them, to such cold temperatures, which are generally considered uncomfortable and potentially dangerous. However, occasions arise in which a consumer and a consumer electronic device are at cold temperatures near or below freezing. If the consumer electronic device is an inactive state, then any current draw from the battery is typically so low that the effects of the low temperature are generally negligible. However, shifting the consumer electronic device to an active state, e.g., by trying to place a call with a mobile phone, may require significant current flows, which may be unavailable at low temperatures, or may cause significant voltage drops and subsequent brown outs for the control circuits that can result in the shutdown of the consumer electronic device.